1. Field of the Invention
The present invention relates to an image pickup apparatus equipped with an autofocus adjustment mechanism for automatically performing the focus adjustment of an image pickup lens by performing distance measurement up to an object, a control method of the image pickup apparatus, a control program of the image pickup apparatus, and a storage medium.
2. Related Background Art
As a conventional focus adjustment method, there is a known method named as a hill climbing method, by which a lens is moved toward a direction in which high frequency components of a luminance signal (hereinafter referred to as evaluation values) obtained from an image pickup element increase and then a lens position at which the evaluation values take the maximum value is regarded as a focused position. In the hill climbing method, as shown in FIG. 7, the midsection in a photographed picture of a camera is ordinarily set as a distance measurement area, and a lens position at which the evaluation values of the object in that area take the maximum value is regarded as the focused position. The relation between the lens positions and the evaluation values which are obtained in such a way takes a shape of a mountain, as shown in FIG. 4.
However, the hill climbing method has a defect such that a time necessary for focus adjustment becomes longer in proportion to the number of stopping positions of a focus lens. For example, in the case where an optical system having a long focal distance and a high magnification, the time necessary for the focus adjustment is strongly affected by the number of the stopping positions.
As means for settling such a problem, there is a method which has a plurality of patterns of obtaining periods of evaluation values in obtaining operations of the evaluation values (hereinafter referred to as scans) while using the above-mentioned hill climbing method. For example, a method (disclosed in, for example, Japanese Patent Application Laid-Open No. 2002-318341) is used in which, as shown in FIG. 4, a long period scan is first performed to specify, on the basis of the results thereof, a position of the focus lens at which the evaluation values take a peak in the long period scan (hereinafter referred to as a provisional focused position), and then a short period scan is performed in the vicinity of the provisional focused position to specify a final focused position.
On the other hand, in case of adopting the above-mentioned autofocus adjustment method of obtaining the focused position using the combination of the long period scan and the sort period scan, there is a case where, even if a lens position having a peak of the evaluation values has been able to be specified by the long period scan, a focused position cannot be specified by the short period scan in the vicinity of the peak. This case is, for example, one in which no peak of the evaluation values is suitably obtained since the object does not stay in the used distance measurement area owing to vibration or the like at the time of the short period scan. Moreover, a method of setting the focus lens at a hyperfocal distance is known as a setting method of the focus lens in the case where distance measurement is judged to be impossible. In case of adopting this method, the photographing distance becomes the hyperfocal distance (the photographing distance=the hyperfocal distance). And then, the front side of the depth of field is at a position of ½ of the photographing distance, and the rear side of the depth of field includes up to infinity. Thus, focus is almost adjusted.
However, an image which can be obtained by performing photographing with the focus lens being focused at the hyperfocal distance has a feeling of a low resolution, and is an image which is impressed to be out of focus as the whole. Moreover, it is a matter of course that, when an object is located on the side nearer to the half of the hyperfocal distance in the case where the focus lens is positioned at the hyperfocal distance at the time of being out of focus, a photographed image becomes a defocused one. Consequently, an autofocus adjustment apparatus capable of obtaining an image having a resolution as high as possible even in the case where the focus lens is judged to be out of focus has been desired.